Water-soluble chromium(VI) may cause irritation or allergic reactions when placed in contact with the skin, and is classified by the European Union as a carcinogenic substance. Exposure to it should therefore be kept to a minimum. European Directive 2003/53/EC of 18 Jun. 2003 therefore prescribes for cements a chromium(VI) content of less than 2 ppm.
Additives are known which allow cement to be treated to reduce chromium(VI). Compositions containing Fe(II) sulphate as an agent for reducing chromium(VI) may be used to this end. However, Fe(II) is unstable in aqueous solution, so these additives have to be used in powder form. A precise dosage of powders is awkward and requires specific equipment. Moreover, these additives rapidly lose their capacity to reduce chromium(VI) ions, owing to oxidation of the Fe(II) ions on contact with air. The efficacy of these additives is therefore reduced if they are stored for a long time.
In a highly basic medium (pH≧13), such as in the interstitial medium of cement, tin(II) reacts with the hydroxide ions of the medium so as to form the Sn(OH)42− ion according to Equation 1.Sn2++4OH−→Sn(OH)42−  Equation 1
The Sn(OH)42− ion is able to reduce CrO42− to Cr(OH)3− by the oxidation/reduction reaction of Equation 2.2CrO42−+8H20+3Sn(OH)42−→2Cr(OH)3+4OH−+3Sn(OH)62−  Equation 2
It will be recalled that the redox potentials of the pairs used during the reaction are as follows:E0(Sn(OH)62−/Sn(OH)42−)=−0.96VE0(CrO42−/Cr(OH)3=−0.12V
Thus, a colloidal suspension of tin(II) hydroxide stabilized by a stabilizing agent and optionally by a thickener is known from WO 2005/016843.
The tin(II) ions tend to oxidise to tin(IV), thus reducing the efficacy of the additive. Furthermore, the solubility of the salts of tin(II) ions is satisfactory only at very acidic pHs. Now, solutions having a pH of less than 2 are considered to be corrosive, and the use thereof requires expensive suitable equipment.
Besides, an agent for reducing Cr(VI) ions in hydraulic compositions comprising tin(II) ions, in synergistic association with a lignosulphonic acid, is known from EP 0960 865.